Armor reduction is the effect of some items and champion abilities that temporarily reduces the armor of one or more targets. This allows allies to benefit from the reduction of armor. This can reduce armor below 0, resulting in bonus damage (see Armor).

Order of calculations

When attacking, armor penetration and armor reduction are considered in the following order [1]:

Percentage armor reduction makes a bigger difference on targets with higher armor. For instance, with 40% armor reduction, a target with 200 armor will lose 80 while a target with only 50 armor will lose 20.

Percentage armor penetration

The target's armor is treated as being multiplied by a percentage for purposes of damage calculation (100% − the listed penetration). Percentage armor penetration stacks multiplicatively and is ignored if the target's armor is 0 or less.

Example: 20% armor penetration causes the target to take damage as if its armor was multiplied by 80%, so a target with 30 armor will be treated as though it had 24 for purposes of damage calculation (the target's actual armor value will not change, however).

Because the target's actual armor value does not change, anything else depending on that value also doesn't change (e.g. Rammus'sSpiked Shell).

Percentage armor penetration makes a bigger difference on targets with higher armor. For instance, with 40% armor penetration, a target with 200 armor will be considered as having 80 less while a target with only 50 armor will be considered as having 20 less.

Flat armor penetration

The target's armor is treated as being reduced by an amount for purposes of damage calculation, but cannot be reduced below 0. Flat armor penetration stacks additively.

Example: 20 armor penetration causes the target to take damage as if its armor was 20 less, so a target with 30 armor will be treated as though it had 10 for purposes of damage calculation (the target's actual armor value will not change, however).

Because the target's actual armor value does not change, anything else depending on that value also doesn't change (e.g. Rammus'sSpiked Shell).

Examples

Given 20 flat armor reduction and 30% armor reduction, and the target is affected by 10 flat armor penetration and 35% armor penetration,

Target A has 80 armor.

The 80 is reduced to 60 by the 20 armor reduction.

The 60 is reduced to 42 by the 30% armor reduction.

The 42 is considered to be 27.3 by the 35% armor penetration.

The 27.3 is considered to be 17.3 by the 10 armor penetration.

Target A takes damage as if it has 17.3 armor.

Target B has 18 armor.

The 18 is reduced to −2 by the 20 armor reduction.

The −2 is not affected by any further calculations because it is less than 0.

Trundle'sSubjugate reduces the armor of his target by 20% and gains for the same amount. This amount is doubled over the next 4 seconds. The armor reduction lasts for another 4 seconds after the drain completes.